Capacitors have proven to be an effective low cost means for controlling power factor in electrical distribution and transmission systems. The problem at hand is to provide the best possible capacitor protection, to optimize capacitor usage, and to minimize or eliminate circuit outages associated with capacitor installations. In essence, this means providing coordinated protection to maximize capacitor utilization and to minimize the effects of capacitor failure or rupture.
When a power-factor correction capacitor fails, it is essential that the unit be disconnected from the associated electrical network. If the failing capacitor is not disconnected within a relatively short time, the capacitor tank may rupture. Capacitor tank failure can result in the discharge to the environment of hazardous materials, such as polychlorinated biphenyl's (PCB's). Tank rupture prevention is also important since a tank rupture can result in a fire. This is because some capacitors contain a combustible dielectric fluid.
Substation capacitor banks are normally equipped with individual capacitor unit fuses. To operate successfully, a capacitor fuse must interrupt the capacitor unit circuit after the capacitor fails, but before the capacitor tank ruptures. This requirement of tank rupture prevention is of particular importance because of the environmental and safety issues often associated with the materials used in capacitors.
In certain applications capacitors can be safely fused with indicating-type expulsion fuses. Individual expulsion fuses for capacitor banks are less costly than current limiting fuse installations. An example of an early expulsion fuse is presented in U.S. Pat. No. 2,096,983. These fuses are preferably bus-mounted, indicating-type expulsion fuses. Examples of such fuses are shown in U.S. Pat. Nos. 4,121,186 and 4,275,373.
Power-factor capacitors are normally installed in a horizontal array or bank. They are either pole-mounted, mounted upright or along their sides in a single row (e.g., FIG. 1 of U.S. Pat. No. 4,121,186) or multitiered array, or in an outdoor metal enclosure. In each case the individual capacitor units are ganged together leaving sufficient space for natural-air circulation and cooling. When the capacitor units are closely spaced to each other, the use of individual bus-mounted, indicating-type expulsion fuses can become a source of difficulty. Those skilled in the art know that an indicating-type expulsion fuse link includes: a flexible lead or "fuse leader" (sometimes called a "pig-tail"), a fusible element, a ferrule or "button", and a tubular enclosure. The fuse leader is connected to one end of the fusible element. The other end of the element is connected to the ferrule which is carried by the fuse housing. The fusible element, a portion of the ferrule and the fuse leader are contained within the tubular enclosure (sometimes referred to as the "auxiliary tube"). This fuse link is then assembled within a hollow insulated tube (sometimes called the "fuse tube"). When fault current is initiated, the temperature of the fusible element rapidly increases and melts. The current flowing through the fuse then bridges a tiny gap and an arc is formed. The arc grows and expands rapidly. The arc heats the internal surface of the auxiliary tube causing it to decompose and produce gas. The turbulent high pressure gas thus developed tries to break-up the arc. As the arc and gas resist each other, an equilibrium arc position is established. The pressure in the tube builds up rapidly and the gas begins to vent from the bottom of the fuse tube. During this same time the arc length increases as the gas blows the fuse leader out of both tubes. As current zero approaches, the diameter of the arc diminishes and eventually disappears. However, the fuse leader is expelled from the fuse tube at high velocity. Under some conditions, the free end of the fuse leader has been known to be thrown against the adjacent leaders connected to the capacitors on either side of a capacitor unit that suffered an over-current condition. This can cause a cascading effect resulting in multiple fuse blowings and can contribute to early failure of the associated capacitors.
It is relatively common practice to provide a spring-like device connected to the exposed end of the fuse leader when an indicating-type expulsion fuse is used. (See U.S. Pat. No. 4,275,373) This device facilitates the removal of the fuse leader from the fuse tube when the fuse operates. The spring also tends to bias or hold the exposed end of the flexible fuse leader away from the adjacent capacitor units. These spring-like devices are in some cases simple coil springs; in other cases they are essentially flat pieces of metal or leaf springs, the plane of which is oriented generally perpendicular to the longitudinal axis of the insulated tube at the end of the expulsion fuse. However, in most cases these spring-like devices are relatively flimsy and do little to control the whipping of the fuse leader following expulsion. One solution to this problem is described in Soviet Union Invention Certificate SU 936-070 which was published on June 15, 1982. That certificate describes a "link catcher". The link catcher is made from a flat ring with its center disposed along the axis of the fuse tube. The ring, as such, catches the free end of the fuse leader and prevents tangling of contacts or the striking of adjacent members. This prevents adjacent fuses from actuating and multiple capacitors from being taken out of service. This device, however, is very dependent upon the alignment between the end of the fuse tube and the center of the link catcher. Another approach to the problem is described in U.S. Pat. No. 3,783,342. There an integral cage is used to control the release of debris when the fuse actuates. Neither invention can be said to be a positive or active means for leader whip control.
Considering the importance of capacitor banks and the eventual consequences of several fuses blowing in the same bank, a reliable design is needed for a device to minimize the effects of leader whip following the operation of an expulsion fuse. Such a design will go far to improve capacitor reliability, the protection of the environment and the efficient use of electricity.